When making lenses by a method of this kind, it is known to take advantage of the ability possessed by plastics materials to deform when heated in order to impart the desired shape to said lenses. Starting from a mass of thermoplastic material of standard shape (commonly referred to as a “parison”) or from a special preform of a shape that is close to the desired shape, this shaping is performed by forming under pressure between two dies that are pressed by means of a press against the material that has previously been heated to the plastic state. The dies are generally made of stainless steel and each has a forming face possessing suitable curvature(s), corresponding to the curvature(s) to be given to the finished lens, and presenting optical polish, i.e. analogous to a mirror polish.
To obtain lenses possessing optical qualities suitable for their function, various precautions must be taken during manufacture, in particular in order to avoid irregular deformations or the presence of internal tensions, said deformations or tensions giving rise to anisotropy or to undesirable aberrations.
In this respect, special care is taken in making molding dies. Furthermore, it is often recommended to perform manufacture in two successive stages: firstly a blank is made to dimensions that are close to those of the desired lens, e.g. by injection molding, forming, or machining, and then the blank is finished by hot-forming.
These precautions relating to the tooling and the method of operation must also be associated with precautions relating to the heating of the plastics material and of the mold during forming. Ordinarily, heating is provided by the dies themselves, which dies are arranged to receive a heating fluid or gas, or electrical resistances. The heat provided by hot fluid flow or by electrical resistances spreads throughout the mass of the dies and is communicated by the walls thereof to the plastics material while it is being formed.
Heating in that way presents drawbacks in practice.
Firstly, it does not ensure that the plastics material is heated in regular and uniform manner as would be desirable to avoid internal tensions. Different portions of the dies and, as a result, of the material to be shaped, are brought only progressively and unevenly to the temperature required for forming. This lack of uniformity in the transmission of heat also occurs during cooling after forming.
This unequal heating and cooling at various points of the shaped material causes the resulting optical lens to retain deformations and tensions that often make it unsuitable for its intended optical purposes.
Furthermore, in usual arrangements, the dies for receiving the heating and cooling devices are fixed on the press, and it is in consideration of this arrangement that connections are provided with heating fluid feeds or with electricity feeds. With installations of that kind, the operations of installing and removing dies are lengthy and complicated.
Another method of heating that seeks to mitigate those drawbacks consists in using external heater means that are not part of the mold itself, and in particular not part of the dies. The mold containing the preform or parison is thus heated separately from the press prior to being installed therein for the forming operation. Nevertheless, the fact of the mold being heated separately from the press by external means, although it does indeed enable the dies to be free of any hydraulic or electrical connection, nevertheless suffers from the major drawback of requiring the mold to be handled in each cycle, thereby considerably slowing down production throughput. In addition, the temperature of the mold cannot be controlled during forming and no provision is made for cooling the mold after forming, so like heating, cooling can be envisaged only by using external means, thus requiring further handling.
In order to enable the mold to be taken from a heating station to the forming station (i.e. the press), the mold has a sheath covering the dies and within which the dies are slidably mounted. The function of the sheath is then to ensure that the mold constitutes a mechanical unit by providing a mechanical connection between the two dies independently of the press.